C-reactive protein (CRP) is a component of innate immunity and whose serum level rises during inflammatory states including bacterial infections. In vitro, CRP binds to cell wall C-polysaccharide (PnC) on Streptococcus pneumoniae and subsequently activates the complement system in serum. CRP also binds to complement factor H, the protein that pneumococci also bind to and use to escape complement-mediated killing. In murine models of infection, human CRP is protective against lethal infection with S. pneumoniae. Our long-term goal is to define the mechanisms by which CRP protects against pneumococcal infection in mice. How does CRP, directly or indirectly, act on the bacterial surfaces to kill them? Our hypothesis is that the mechanism of protective action of CRP involves the activation of the complement system. It was assumed that CRP was protective through a pathway in which CRP binds to PnC, activates complement through the classical cascade, and then bacteremia is reduced through complement-dependent phagocytosis. Two observations suggest that this pathway is not sufficient. The mechanism of CRP protection is much more sophisticated than previously appreciated. First, human CRP cannot bind murine C1q and therefore cannot activate the classical complement cascade. Second, a CRP mutant incapable of binding to PnC is still protective against pneumococcal infection in mice. Each of these two intriguing observations will be separately pursued in the following two specific aims. 1. To test the hypothesis that the activation and recruitment of the complement components on the pneumococcal surface, subsequent to the binding of CRP to pneumococci, participate in CRP-mediated protection of mice from pneumococcal infection. 2. To test the hypothesis that the binding of CRP to complement factor H on factor H-coated pneumococci participates in the protection of mice from infection. PUBLIC HEALTH RELEVANCE Our goal is to understand the mechanisms by which the in vitro binding and functional capabilities of C-reactive protein (CRP) relate to its in vivo functions in inflammation. Elucidation of the mechanisms by which CRP protects mice from Streptococcus pneumoniae infections would help achieve our goal. In addition, the investigation of CRP-complement factor H interactions may also have implications in other areas of clinical medicine such as age-related macular degeneration.